You can definitely take one (but only one as you suggested) through the First Wing, as access to the First Wing is dependent on eligibility for Galleries First lounge access.

Theoretically you could all check in at the First checkin, but then at least 3 of your party would need to go through normal or Fast Track security (if you can persuade them to let you through Fast Track - you could get one guest per cardholder through Fast Track but officially not the other person)

For boarding, in my experience they do check that others in your party are on the same booking and I have seen groups split up when they're not on the same booking. I don't know if this is official policy, but I have certainly seen it happen.